This comes with the switching costs: it takes longer to finish two tasks done simultaneously (switching rapidly) than one after the other (Marois & Ivanoff, 2005, Dux et al, 2006).
Actual multitasking is only possible with activities that are well-learned and don't take up much of our working memory when we perform them – they don't need conscious attention. E.g. driving a car. These are "automatic processes"
Automatic processes usually involve movement, while "effortful processes" involve processing of verbal information and logic.
We can perform two automatic processes at once, or one automatic process and one effortful process at once, but not two effortful processes at once.